Using dispersants to break up oil leaking from the Rena may cause more harm than good to marine life, an environmental consultant says.

Rick Steiner, a veteran of the Gulf of Mexico spill last year, said the dispersant Corexit 9500 was not suited to near-shore waters.

Mr Steiner, based in Alaska, said dispersants were supposed to break up the oil sitting on the surface and take it down into the water column.

"If it's in shallow water then it's going to contact the seabed habitat and then foul that and that's somewhat what happened in the Gulf of Mexico.

"Plankton, fish and shellfish larvae are down 10-60m deep into the water which are all of a sudden exposed to this toxic chemical soup."

The decision to use a dispersant was often about public relations, he said.

"The choice to use dispersants in a major offshore spill is usually made by the industry and/or government because they want to do away with the public relations disaster the spill is causing.

"They want to get the spill off the surface, down into the water column and out of sight - and to their minds out of mind."

A Maritime NZ spokesman confirmed Corexit was sprayed over the spill on Thursday and Friday on a "trial basis" but its effectiveness had been inconclusive.

There were plans to try again yesterday but that was hindered by the weather.

Mr Steiner said it was not easy dealing with fuel spills.

"The oil is pretty vicious, it's not necessarily as toxic as some of the other petroleum products or crude oil but it's very heavy and difficult to deal with and it's very fouling to bird feathers and mammal fur."

He said strong winds would naturally break the oil up but would make it more difficult to recover.